Auto Login for C# Desktop Application

I am developing an application for Windows Mobile that needs to access Facebook. I am able to do everything that I need to do quite easily using the Facebook.Compact library except authentication.

Every time that I start the application, the user is forced to sign in to Facebook even when they click the remember box. I would like to have them sign in once and I remember the AuthToken and SessionID and do an Auto Login thereafter. I do this
quite easily for Flickr for example.

I have tried various combinations of saving and restoring the AuthToken, the UserID and the SessionID as well as fbAPI.ConnectToFacebook() and fbAPI.CreateSession(AuthToken) but I do not seem to be able to discover the right way to do this. Each attempt
gets an error like Invalid Signature or Invalid Parameter. I see lots of documentation for the case of ASP be I cannot find how to do it with a desktop Windows Mobile application.

Can anyone please point me at the right place to learn how to do this?